In magnetic disk recording, it is well known that customer information data bits are recorded as transitions of magnetization along a disk track. If the data bit is a "1", then a magnetization transition is recorded (say, from north to south). If the next data bit is also a "1", another magnetization transition is again recorded (this time from south to north). On the other hand, if the data bit is a "0", then no transition is recorded. Crowding of transitions causes intersymbol interferences (ISI) and makes data recovery more difficult due to resolution limitation in the readback head. As the maximum data density of a storage device is limited by the amount of intersymbol interferences that can be tolerated by its read/write channel, the traditional approaches to increase storage capacity of disk storage devices have been directed to overcoming the ISI limitation. One of these approaches is partial response maximum likelihood (PRML) detection, which seek to improve the detection capability of recorded signals in the presence of noise.
Another way to alleviate crowding is to use a code that requires n "0's" between consecutive "1's" to thereby reduce transition crowding by a factor of n+1, and codes that have this property are called run-length-limited (RLL) codes. Examples of such codes are the (2,7) code, which requires two "0's" between consecutive "1's" and the (1,7) code which requires one "0" between consecutive "1's".
RLL codes alleviate transition crowding by introducing redundant "0's" to prevent the occurrence of consecutive "1's". Hence, the number of encoded bits is greater than the number of customer bits, the difference being the number of redundant bits. However, while RLL codes introduce redundant bits into a string of customer information bits, they increase the overall effective storage capacity of a recording device by allowing, as the result of such introduction, transitions to be recorded further apart. The rate of a code is defined as the ratio of the number of customer bits to the number of encoded bits. The (2,7) code has a rate of 1/2 and the (1,7) code has a rate of 2/3. Magnetic recording systems today use one code throughout the disk recording surface, and also record the same number of bits on each track. However, because the inner tracks are shorter, the transitions are more crowded on the inner tracks than on the outer tracks.
The industry is seeking to increase data capacity by making the density of transitions more uniform throughout the disk surface. "Constant density recording comes alive with new chips", Electronic Design, Nov. 13, 1986, p. 141, is evidence of the approach currently being taken. The article suggests using a higher clock frequency for outer tracks, hence increasing the number of transitions recorded there and the overall disk surface capacity. Because of the higher clock frequency used on outer tracks, more bits are recorded as the disk rotates one revolution.
Varying the frequency in this manner presents some engineering problems in designing the channel to handle a range of frequencies. Also, synchronous DASD architectures are less amenable to variable bit rates than nonsynchronous architectures. Other attempts include the suggestion of making outer tracks narrower than inner tracks; or varying the rotational speed of a disk faster as a function of increasing track radii, so that a constant rate of transition can be recorded throughout the disk surface. To those skilled in the art, however, it is readily clear that these approaches are inefficient because of the complicated mechanical design and implementation involved.
An object of this invention is to increase disk storage capacity by exploiting track length differences between outer tracks and inner tracks. It is another object of this invention that such increase be accomplished with only minimal impact on the design, such as using only a single clock frequency, and manufacturing of the disk device.
This invention accomplishes uniformity of transition density throughout a disk surface by keeping the same clock frequency, but changing the code used. Outer tracks use higher rate codes than inner tracks. That is, the code used on outer tracks requires fewer "0's" between "1's". The use of higher rate codes on the outer tracks tends to equalize the transition densities between the outer and inner tracks. And, because higher rate codes encode more customer bits per transition, the outer tracks contain more customer information than the inner tracks. The result is an increase in disk surface capacity and a more uniform transition density throughout the surface.
According to this invention, the method in which the above objects are satisfied comprises the steps of: partitioning recording tracks of the device into a plurality of recording bands, including a first band surrounded by an outer band; encoding data being recorded on said first band under a first run-length-limited code; and encoding data being recorded on said outer band under a second run-length-limited code having a higher code rate than said first run-length-limited code.